There has been known an electron beam apparatus for scanning with an electron beam(s), a surface of a semiconductor wafer which is a sample to be inspected, detecting secondary electrons emitted from the wafer, generating image data of the wafer surface based on obtained detection signals, and detecting matching or mismatching of images of dies on the wafer to detect a defect(s) on the wafer.
As one of electron beam apparatuses as above, an apparatus using a mapping projection type electro-optical system has been also known. In the mapping projection type electron beam apparatus, secondary electrons or reflection electrons which are emitted from a surface of a wafer by the irradiation of a primary electron beam, are magnified and imaged by a multi-stage lens system including an objective lens. Uniform irradiation of an electron beam can be made to a relatively large area on the sample surface, so the inspection can be performed in higher throughput than that of an SEM system.
In a prior electron beam apparatus, the following focusing method is employed.
An auto-focus map (AF-MAP) is created every time a wafer is loaded onto a stage of the electron beam apparatus. When the sample is to be inspected, a voltage applied to electrodes of an electrostatic lens for focusing, is adjusted based on stage coordinates and data of the AF-MAP. Therefore, even when a position on the wafer surface in an optical axis direction is fluctuant, image data can be obtained under a condition that focus is achieved at any time.
The creation of the AF-MAP is executed as follows, using a sample inspection electron beam apparatus as a focus detection device.
At first, measurement points are set on a wafer, and a stage on which the wafer is placed is moved to a position so that a measurement point locates below the focus detection device. Although it is preferred to detect a focus value at a regular measurement point (for example, a lower left point of each die) on the wafer, a focus value may be detected at intervals of n dies and appropriate complimentary processing is executed to obtain focus values for dies focus values of which have not been detected. Alternatively, focus values at a plurality of measurement points of each die may be detected. Also, a measurement point may be selected at random. After detecting the focus values for the measurement point, the best focus value for the measurement point is determined and stored.
The stage is moved again to a location so that a next measurement point locates below the focus detection device to detect focus values. Then, the best focus value for the next measurement point is determined from the measurement values and stored. Therefore, the best focus values are determined and stored for all measurement points to create the AF-MAP.